Electrodeposition of palladium



United States Patent 3,290,234 ELECTRODEPOSITION 0F PALLADIUM Edward -A. Parker, Cranston, R.I., and Alfred M. Weisberg, Chicago, Ill., assignors to Technic, Inc., Cranston,

R.I., a corporation of Rhode Island N0 Drawing. Filed Oct. 29, 1963, Ser. No. 319,661 4 Claims. (Cl. 204-47) The present invention relates to the electrodeposition of palladium and, more particularly, to the preparation of palladium electroplates possessing bright, smooth, and lustrous surfaces even at heavy thicknesses.

The known methods of depositing palladium from bath used in the prior art suffer various difliculties. Among these may be mentioned instability of the bath, buildup of deleterious side-products, hazy deposits in all but thin coatings, and low deposition rates.

It is therefore a primary object of the present invention to provide a solution from which. palladium may be electrically deposited in a convenient maner whereby a plate is produced which is bright, smooth, and lustrous.

It is a further object of the present invention to provide a solution from which palladium may be electrically deposited at room temperature.

' It is another object of the present invention to provide a solution from which palladium may be electrically deposited in thick smooth plates at relatively rapid rates.

It is still another object of this invention to, provide a process for the aqueous electrodeposition of palladium from a neutral solution. I

Other objects, advantages, and results of the present invention will be apparent to anyone skilled in the knowledge of this art.

We have discovered that these several advantages can be secured by electroplating from a bath containing palladium in the form of a complex in solution in the concentration in the range of 2-10 grams of metal per liter of solution in the form of a complex of a cyclic-alkylenemoiety diamine tetraacetic acid, for example, selected from the group consisting of cyclopropylene, cyclobutene, cyclopentene and cyclohexane, N,N -tetraacetic acid. Also, it should be understood that the acetic acid moiety can be replaced by propionic acid moiety. More generally stated, the chelate complex may be defined as follows:

wherein the X is a cyclic aliphatic moiety having Ns on adjacent carbon atoms as the diamine wherein acetic acid moieties replace hydrogens to form the amino acid chelating compound.

The advantage of these cyclic moieties is that the chelate formed with palladium is strong enough to hold the palladium in solution. In other forms of chelate it is not stable and is reduced to the elemental state.

It is known that palladium forms very strong chelates with aminopolycarboxylic acids. With common ligands of this type, such as ethylenediamine tetraacetic acid, the reducing power of the amino acid moiety is sufiicient to reduce aqueous solutions of the palladium chelate to the elemental state. It has been found that this reduction reaction is accelerated by a low pH.

We have discovered, however, that the aqueous solution of a palladium N,N cyclohexanediamine tetraacetic acid chelate is stable over the pH range of 4 to 12. Below pH 4 the limited solublity of the chelate 3,290,234 Patented Dec. 6, 1966 practically precludes its use as an effective electroplating solution.

Similarly, other diamines having a cyclic moiety between the nitrogens are stable in the broad pH range and are responsive to electric currents so that our fundamental conditions are preserved, namely, the palladium is kept in solution and is electro-deposited, and contaminating metal ions are precipitated.

The palladium metal concentration in this bath may be varied over wide limits, but a practical range for clean, mirror bright plates is 2 to 10 grams of metal per liter.

The temperature of this novel bath may be varied from to 65 C. It has been found, however, that room temperature operation is a facile manner to obtain bright, stress-free deposits.

The pH of this bath is not critical between the limits I of 4.0 and 12.0. A neutral bath with a pH range of 6 per, iron and the like appear in the bath as a result of drag-in, etc. Too much contamination by such metal ions in solution will ruin the usefulness of the bath be cause the contaminating metal'will plate out. Under the conditions we maintain, particularly, the strong complex of palladium, acid pH, and soluble phosphate in solution, without other acids having good complexing properties, contaminating metals are precipitated as they reach the plating bath.

Cathode current efficiences are quite high which tends to eliminate any stress due to hydrogen embrittlement. A current efiiciency approaching may be obtained at a current density of 10 amperes per square foot. Higher metal concentrations permit the use of higher current densities. A similar result may also be obtained with the aid of agitation.

Insoluble anodes of stainless steel, platinum, or carbon may be used. It is important to avoid anodes of any kind which, because of the chemistry of the solution, are likely to contribute undesirable ions in the plating bath. The palladium metal concentration may be maintained on the basis of ampere hours of use of the bath and by the addition of palladium as oxide or cyclohexene diamine tetraacetic acid salt to replace losses. With the removal of substantial amounts of palladium from the bath, it is possible to avoid accumulation of excess free chelating agent in the bath by the addition of palladium oxide to be dissolved and to form the chelate.

The following examples illustrate the application of these principles to the bath composition and operation, and the exemplification which follows should not be construed as limiting the invention but merely illustrates its spirit and scope.

Example I 5 grams of palladium as the palladium chelate of N,N

cyclohexane-diamine tetra-acetic acid 60 grams of mono-potassium phosphate.

Water to one liter.

This bath when used at 50 C. and a current density of 5 ASP will produce a brilliant palladium electrodeposit.

Example 11 10 grams of palladium as the palladium chelate of Nb",

cyclohexane-diamine tetra-acetic acid 40 grams of mono-potassium phosphate 30 grams of citric acid 3 Water to one liter.

When this solution is adjusted to pH 5.0, pure, mirror bright palladium electrodeposits may be obtained at room temperature with a current density of 5 ASP.

Example 111 Using the fomulae of Examples -1 and II, the chelate of cyclopropylene diamine tetra-acetic acid is useful.

Example IV Using the formulae of Examples I and II, the palladium chelate of cyclobutene N-N diamine tetra-acetic acid is useful.

Generally, a palladium concentration in the range of about 2 grams to about grams of the palladium metal per liter of solution is most useful.

In carrying out plating operations using these baths, it is to be noted that good plating technique is to be followed. That is, ampere hours of use of the bath are measured, the temperature of the bath is controlled reasonably closely to the recommended levels, it is agitated, pH of the bath is followed and adjustment within reasonable tolerance of recommended levels is made and, generally, clean plating techniques are followed. Regardless of the care with which plating operations are carried on in a working installation, it may be noted that when a base'metal such as iron ha been plated with copper and nickel, whether the article he costume jewelry or a piece of scientific apparatus, the best techniques will still produce situations in which the plating baths become contaminated with iron, copper, nickel, etc. Since it is not the desire of the operator to plate palladium alloys, this bath automatically corrects such contamination conditions, because by maintaining the palladium in platable condition in the form of the highly selective chelates described and maintaining acid conditions in the bath at a level such that the phosphates of iron, copper, nickel and the like are directly precipitated as such, the contaminants are automatically precipitated. Hence, the bath, for all practical purposes, is a bath of pure palladium chelate. For good operating technique, it is desirable occasionally to filter the bath.

The foregoing examples illustrating working formulas for the deposition of bright palladium coatings specifically indicate an operating pH of 5. It is to be understood that operable pH within the range of 4-12 can be used with the palladium chelate solutions identified but the preferred range is slightly on the acid side, 567.

One of the reasons for the acid pH operation is to maintain conditions in the bath such that contaminating metal ions will he precipitated as their phosphates, Accordingly, all of the examples may be actually read as pH operation 4-12, preferred being 56-7. Similarly, where the specific examples show the cyclohexene diamine acetic acid compound as the chelating agent, it should 'be understood that substantially the same results are obtained when the cyclopropene, cyclobutene, cyclopentene are used, because the stability of these compounds is of the same order of magnitude as the cyclohexene.

What is claimed is:

1. A bath for electro-plating palladium consisting essentially of the palladium chelate of an N,N -cyclic alkylene diamine tetraacetic acid dissolved in water with a phosphate conductor, and a buffer to hold pH at a given level in the range about 4-12.

2. A bath in accordance with claim 1 in which the palladium compound is the palladium chelate of cyclo hexane diamine tetraacetic acid and the bath is buttered to a pH in the range from about 4 to about 12.

3. A bath in accordance with claim 2 in which the concentration of palladium compound maintained in the solution is in the range from about 2 grams to about 10 grams of metal per liter.

4. A method of electro-plating palladium to form heavy, clean, stress-free, bright plates which comprises providing an aqueous solution of palladium in the form of a chelate stable under plating conditions, the said chelate consisting essentially of palladium chelates of cyclic aliphatic N,N -diamine tetraacetic compounds in the presence of anions in said solution suitable for precipitating contaminating metal but insuflficient to precipitate the palladium, said solution being maintained at a pH in the range from 4-12, and passing an electric current through said solution from an anode to a cathode to be coated.

References Cited by the Examiner UNITED STATES PATENTS 2,335,821 11/1943 Wise et al. 204-47 2,452,308 10/1948 Lambros 20447 2,984,595 5/1961 Schumpelt et al. 204-47 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

4. A METHOD OF ELECTRO-PLATING PALLADIUM TO FORM HEAVY, CLEAN, STRESS-FREE, BRIGHT PLATES WHICH COMPRISES PROVIDING AN AQUEOUS SOLUTION OF PALLADIUM IN THE FORM OF A CHELATE STABLE UNDER PLATING CONDITIONS, THE SAID CHELATE CONSISTING ESSENTIALLY OF PALLADIUM CHELATES OF CYCLIC ALIPHATIC N,N1-DIAMINE TERAACETIC COMPOUNDS IN THE PRESENCE OF ANIONS IN SAID SOLUTION SUITABLE FOR PRECIPITATING CONTAMINATING METALS BUT INSUFFICIENT TO PRECIPITATE THE PALLADIUM, SAID SOLUTION BEING MAINTAINED AT A PH IN THE RANGE FROM 4-12, AND PASSING AN ELECTRIC CURRENT THROUGH SAID SOLUTION FROM AN ANODE TO A CATHODE TO BE COATED. 